1. Field of the Invention
The present invention relates to a radio system, and in particular, though not exclusively, to a quasi-synchronous radio system.
2. Related Art
A simplified example of a quasi-synchronous, bi-directional radio system is shown schematically in FIG. 1, where a control centre 1 is connected by fixed inks (A, B, C, D) to transceiver (sometimes referred to as transmitter) sites 2, 3, 4, 5. The system of this example is bi-directional permitting two way communication between a mobile 6 and the control centre 1 via the transceivers.
In a quasi-synchronous system transmitter (transceiver) sites all radiate the same signal on nominally the same radio channel. Such systems are suitable for wide area coverage over a limited number of radio channels, sometimes only a single channel, and so are often adopted by police, fire and similar utility services. Geographical considerations and transmitter power restrictions, for example, often mean that it is not possible to achieve coverage of the area served by such services with a single transmitter site, so the multiple transmitter site configuration of a quasi-synchronous or synchronous radio system is appropriate.
In a quasi -synchronous system the radio frequency carriers are within a few Hertz of each other, while the modulation imposed on the carrier remains synchronous. This is necessary because in regions where the coverage of two neighbouring transmitters overlap, a mobile will receive signals from both transmitters. In such circumstances the two signals must be sufficiently synchronous not to destructively interfere. In a synchronous radio system where the signals radiated from all transmitters are identical in all respects and in particular are transmitted on the same frequency, the same circumstances may occur.
In a quasi-synchronous radio system the necessary accuracy of radio carrier frequency can be provided by using accurate ovened oscillators at the transmitters. The synchronisation of the modulated signals between neighbouring transmitters requires, typically, an accuracy of:
AM systems: maximum amplitude differential 3 dB maximum phase differential 30.degree. or 0.018f.degree. whichever is greater (worst case 50 microseconds) PA1 FM systems: maximum amplitude differential 2 dB maximum phase differential 10.degree. or 0.007 f.degree. whichever is greater (worst case 20 microseconds)
To achieve this degree of matching it is necessary that the amplitude and phase transfer functions of fixed links between the control centre and its transmitters differ by no more than these limits across the audio band (300 Hz-3400 Hz). One solution to this problem is to use fixed radio links between the control centre and the transmitters at either VHF or microwave frequencies. These introduce constant delay and can be equalised relatively easily for any differences in their path lengths. Analogue land lines have also been used, but equalisation becomes more of a problem and performance can be poor.
It would be advantageous to be able to use a digital data network to provide the links, for example the Megastream (registered trade mark) service offered by British Telecom which provides customers with 2.048 Mb/s digital paths between pairs of sites. Other examples of digital data networks which could be used are Kilostream (registered trade mark) and ISDN (Integrated Services Digital Network). By connecting appropriate multiplexers at each end, the digital bearer can be used to carry a variety of traffic, including digital coded audio signals. In general with such digital data networks the connection is not direct, but is provided through a trunk network, with customer access being via a local exchange. A characteristic of such networks is that traffic can reach its destination via more than one route, and the network has the ability to re-route traffic to accommodate equipment failures, traffic fluctuations, etc.
This dynamic re-routing facility is such that there is an uncertainty in the propagation delay which will be experienced by a signal travelling between two network terminations. The variation can be of the order of milliseconds which therefore, apparently, rules out any possibility of using such networks to support quasi-synchronous radio directly.